1. Field of the Invention
The present invention relates to an image processing device and method, a program recording medium, and an imaging system.
This application is based on Japanese Patent Application No. 2009-086959, the content of which is incorporated herein by reference.
2. Description of Related Art
A noise reduction method using multiresolution transformation is known for a noise reduction method of reducing noise signals contained in image signals. The relevant method divides the image signal to signals of a plurality of frequency bands using methods of filter bank and Laplacian pyramid, performs some kind of noise reduction process on the signals of respective bands, and re-synthesizes such signals, which method has an advantage in that the noise reduction process of an intensity suited for each divided band can be performed.
FIG. 13 shows one configuration example for realizing the noise reduction process using multiresolution transformation by the Laplacian pyramid. An input image 100 is reduced after being subjected to a low pass filtering process in a filtering unit 101. Such process causes the image size to be halved in both the horizontal direction and the vertical direction. The reduced image is supplied to a filtering unit 111 of the next stage, and further reduced sequentially. The image reduced in the filtering unit 101 is enlarged to an original image size in an enlargement processing unit 102, and a band image signal is generated in a subtractor 103. The band image signal corresponds to a high-pass component shielded by the low pass filtering property used in the filtering unit 101. The noise reduction process is performed on the band image signal including a great amount of high-pass components in a noise reduction unit 104.
Similarly in the process of the second stage, a band image signal corresponding to the second stage is generated by the filtering unit 111, an enlargement processing unit 112 and a subtractor 113, and the noise reduction process is applied on the band image signal in a noise reduction unit 114. The noise reduction process is thus applied on the sequentially band-divided signals. The example shown in FIG. 13 is an example of the processes for a total of three stages.
The output of a noise reduction unit 124 in the lowest frequency band is added with the signal on the low-pass side by an adder 126, enlarged by an enlargement processing unit 115, and then transmitted to the process on the high-pass side. The signals are thus sequentially re-synthesized from the low-pass side, and the signal of the highest pass is ultimately added by an adder 106 to become an output image 130.
In the example shown in FIG. 13, the enlargement process is performed by the enlargement processing units 122 and 125 after the reduction process by the filtering unit 121 of the third stage, but such a reduction process and an enlargement process may be omitted.
Japanese Unexamined Patent Application, Publication No. 2001-57677 discloses a method of applying a noise reduction process (see e.g., Japanese Unexamined Patent Application, Publication No. 55-133179) using a filter with directivity to the noise reduction units 104, 114 and 124 in FIG. 13. Japanese Unexamined Patent Application, Publication No. 9-212623 discloses a method of performing a coring process of having a signal of lower than or equal to a threshold value to zero on the signal of each divided band using a wavelet transformation as frequency division.
The noise reduction method using multiresolution transformation described above has an advantage in that the noise reduction process of an intensity suited for each divided band can be performed, and in that the noise component at lower frequency can also be set as the target of noise reduction by increasing the number of stages of the multiresolution transformation and increasing the number of bands to be divided.